Data storage on magnetic tape is well known and tape recorders have been used to record data tracks on magnetic tape. However, the previous technique to record data in a plurality of parallel data tracks on the magnetic tape was to position the head relative to the tape path by moving the read/write head relative to the tape to different track positions as desired and then holding the read/write head stationary. In such a system, the tape tracks must both be sufficiently wide and separated to guarantee that the exposure of the data track to the head is accurate at least to the minimum requirements necessary to reliably read and write the data. The read/write head is positioned at a predetermined fixed point, relative to the magnetic tape path and the data track must accommodate variations of recording track location and tape location variances as the tape feeds past the head. Historically, this accommodation has been accomplished insuring that the track width and the data track separation on the magnetic tape are sufficient for the read/write head to remain positioned over the designated track and at the same time not read magnetically recorded signals from an adjacent track. This arrangement of track width and track separation will accommodate any deviation of the track location from the design norm either due to being recorded on a first recorder and played or rerecorded on a second recorder or due to the wander of the tape as it is spooled past the read/write head from one spool to the other spool of a cartridge.
The definition of the magnetic read/write head and the track width and separations effectively limits the number of data tracks that may be recorded on any given width of tape. Reliance solely on the track width and track separations to insure reliable read/write operations results in a significant waste of magnetic tape surface and thus limits the data density on the tape.
Whenever the positional requirements of the system are satisfied, the resulting track width and spacing clearly limit the number of data tracks. Data may be recorded in tracks that are much narrower and still be reliable from a read/write standpoint, but the read/write head must be and remain perfectly aligned with the data track. However, as the track width and the read/write head width narrow in an effort to increase the data capacity of a given tape area, any misalignment of the head with the track may lead to read/write repeatability failures and lost data. Thus, the resolution of the head placement mechanism and the precision of the placement of the tape relative to the read/write head become limiting factors affecting the recording density of data on the magnetic tape surface in tape drives having static read/write heads.
In order to overcome the limitation in data density described above, the alignment of the magnetic tape data recording track relative to the read/write head gap of the read/write head must be greatly improved or controlled. Since it is not as practical to attempt to control the tape path and the data track position based on tape position, the choice devolves to controlling the read/write head more precisely and on a responsive basis.
Efforts to control the head of a tape drive on a real-time basis to maintain the head/data track alignment use a servo control to finely position the head relative to the coarse positioning of the stepper motor drive that drives the head carriage to a detented or static position.
In order to position the head precisely relative to a moving data track on tape, a servo read/write head positioning drive is incorporated into the tape drive system. The servo drive parameters of a high density tape cartridge are set forth in a QIC development standard, QIC-139, Rev. G., Aug. 31, 1994, published by Quarter-Inch Cartridge Drive Standards, Inc., 311 East Carrillo St., Santa Barbara, Calif. 93101. Among other criteria this standard defines a pattern of servo tracks and data tracks that must be adhered to insure compatibility of the cartridges recorded on one recorder and read on another recorder. To provide the locational control of the servo read/write head positioning drive, a magnetic read head gap may be placed at a position relative to a servo track on the tape. Then the read head gap is further moved to detect the edge of the servo control track (servo track) recorded on the tape. The read head will provide signals which may be used to indicate the head location relative to the servo track. By using these signals as a basis, the servo control then may produce a positioning signal to drive a servo positioner. The servo positioner moves the read head, causing the read head to track or follow the edge of the servo control track which has been previously recorded on the tape.
Thus as the servo control track deviates from perfect positioning relative to the read head (servo tracking head), the servo control will activate and move the servo tracking head to follow the servo track. The mass of the apparatus used for servo tracking should be as small as possible in order to be responsive and to facilitate very precise placement of the servo tracking head.
The head assembly which incorporates the servo tracking head also has at least one read/write head spaced a precise, predetermined distance from the servo tracking head. Thus, whenever the servo tracking head is moved to follow the servo track, the data heads will be moved a like amount. The relative placement of the data tracks with respect to the servo track is constant, making tracking of the data tracks possible and reliable. Because the tracks, data and servo, are longitudinal to the tape, the tracking motion of the head assembly is perpendicular to the movement of the tape. Movement will be very small in the tracking direction.
Movement in the coarse mode will be in the same direction, perpendicular to tape movement, but in coarse steps that are defined by a stepper motor and an associated mechanical drive.
Additionally, due to the large amount of data that users may have already recorded on tapes using tape drives that do not have servo tracking capability, it is desirable to accommodate the previously recorded data cartridges. Without backward compatibility, a user will be less motivated to migrate to a servo controlled system.
In order to accommodate the older non-servo tracking data cartridges, the servo tracking cartridges must have an identifier so the tape drive can recognize the cartridges and their associated formats.
The tape drive also must have the cartridge recognition capabilities, and the capabilities to disable the servo system and permit the operation of the tape drive in an open-loop mode.
The read/write head of the drive must be accurately and reliably positioned over the data track of the older non-servo compatible cartridges. There being no servo track recorded on the earlier level tapes, the servo mechanism must not function when reading or writing an earlier level data cartridge. The head assembly cannot be allowed to fluctuate, which may be a possibility, whenever not under active servo control. If fluctuations occur, the recorded data tracks on the tape will not be accurately followed by the read/write head; the data may not be reliably read or may not be recorded in such a manner to be reliably read at a later time either with the same tape drive or with another tape drive since the fluctuations of the read/write head cannot be reliably repeated.
The QIC Standard is exemplary of the cartridge and data/servo track recording format which has been published. The servo tracks are grouped in two bands that extend over the recording length of the tape. The QIC Standard is but one of several possible arrangements and only serves to promote compatibility of data cartridges and the tape drives with which they are used.